1. Field of the Invention
The present invention relates to a read-out circuit used for a capacitor-type microphone, and more specifically, to a read-out circuit having a high input impedance, which is applicable to a complementary metal oxide semiconductor (CMOS) process.
2. Discussion of Related Art
In recent years, there has been an explosive increase in the demand for digital apparatuses which receive speech signals such as various mobile phones. Owing to the increased demand for such digital apparatuses, circuit devices, for example, capacitor-type microphones used for the digital apparatuses and preamps configured to amplify output signals of the microphones, have become strongly relied upon.
Digital apparatuses are showing a tendency to be smaller, and thus it is becoming increasingly necessary to downscale circuits used for the digital apparatuses. This has led to a strong need for a System on Chip (SoC) technique capable of integrating circuits on a single chip.
Above all, there is a great need to miniaturize and integrate read-out circuits configured to convert speech signals into electrical signals in digital apparatuses, such as mobile phones.
In general, a read-out circuit may receive a speech signal through a microphone and convert the speech signal into an electrical signal. The microphone may convert the received speech signal into a current signal using a variable capacitance. A microphone using a variable capacitor is referred to as a capacitor-type microphone. Hereinafter, a read-out circuit, which is connected to a capacitor-type microphone and converts an input speech signal into an electrical signal, will be described with reference to FIG. 1(a) to (c).
FIG. 1(a) to (c) show diagrams showing equivalent models of a conventional capacitor-type microphone and read-out circuit.
Referring to FIG. 1(a), the microphone 120 may include a variable capacitor 121, which varies a capacitance in response to a speech signal and generates a current signal. The read-out circuit 110 may include a load resistor RL and a preamp (not shown). The load resistor RL may receive the current signal generated by the variable capacitor 121 and output a voltage signal through an output node 101. The preamp may be connected to the output node 101 and linearly vary the voltage signal.
In this case, the microphone 120 may be an electrical equivalent model of a capacitor-type microphone and may have an intrinsic capacitance Co and a variable capacitance ΔC. The variable capacitance ΔC may be used to generate an electrical signal in response to a speech signal.
The load resistor RL may be used to convert the current signal generated according to the capacitance ΔC into the voltage signal. Here, the current signal generated by the variable capacitor 121 can be expressed as shown in Equation 1:
                              I          C                =                                            ⅆ              q                                      ⅆ              t                                =                                                    V                                  D                  ⁢                                                                          ⁢                  C                                            ·              Δ                        ⁢                                                  ⁢                                          C                P                            ·              2                        ⁢            π            ⁢                                                  ⁢                          f              ·                                                cos                  ⁡                                      (                                          2                      ⁢                      π                      ⁢                                                                                          ⁢                      f                      ⁢                                                                                          ⁢                      t                                        )                                                  .                                                                        (        1        )            where IC denotes a current generated by the microphone 120, VDC denotes a voltage applied between both terminals of the variable capacitor 121 of the microphone 120, ΔCP denotes a capacitance varied in response to a speech signal, and “f” denotes a frequency of the speech signal.
The current generated by the microphone 120 may be converted into a peak output voltage VOPeak, which is expressed in Equation 2, through the output node 101.
                              V          OPeak                =                                            I              CPeak                        ·                          [                                                R                  L                                //                                  1                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                          f                      ·                                              C                        O                                                                                                        ]                                =                                                                                          V                                          D                      ⁢                                                                                          ⁢                      C                                                        ·                  Δ                                ⁢                                                                  ⁢                                                      C                    P                                    ·                  2                                ⁢                π                ⁢                                                                  ⁢                                  f                  ·                                      R                    L                                                                              1                +                                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      f                    ·                                          C                      O                                        ·                                          R                      L                                                                                            .                                              (        2        )            
The current signal generated by the variable capacitor 121, which is expressed in Equation 1, may be proportional to a direct current (DC) bias voltage VDC applied between both terminals of the variable capacitor 121, the capacitance, and especially, the frequency of the input speech signal.
The capacitance varied by the microphone 120 may be proportional to the intensity of the input speech signal. However, on analysis of the characteristics of the voltage signal VOPeak obtained by the load resistor RL shown in Equation 2, a pole is formed in a frequency of Co×RL by the load resistor RL. After the frequency in which the pole is formed, the intensity of an output voltage is proportional to the intensity of an input speech signal irrespective of the frequency of the input speech signal. This characteristic may be obtained using the preamp of the microphone 120.
The preamp of the microphone 120 should linearly vary a voltage signal in a frequency range of about 20 Hz to 20 KHz, which corresponds to the frequency range of a speech signal. Accordingly, in consideration of an intrinsic capacitance Co of a typical capacitor-type microphone, the preamp of the microphone 120 requires a load resistor RL having a high input impedance of several GΩ or higher.
In order to obtain a high input impedance of several GΩ or higher, a resistor having a resistance of several GΩ or higher has been conventionally formed using an additional process. Also, in order to input a voltage signal output by the resistor, a preamp using a junction field effect transistor (JFET) is formed using an additional process.
However, due to various advantages, such as cost reduction, miniaturization, and low power, an integration process has recently involved a standard CMOS process. However, integrating a read-out circuit of a conventional microphone using a standard CMOS process is impossible because a resistor having a resistance of several GΩ or higher and a preamp using a JFET are formed using additional processes other than the standard CMOS process. In other words, integrating the read-out circuit with a digital processing block connected to a rear terminal of the read-out circuit on a single chip is impracticable, thereby precluding downscaling of the conventional microphone and increasing manufacturing cost.